Growth, biomass production, yield, development, morphology, and survival of plants is determined by the growing conditions. Factors affecting these agriculturally important characteristics include, among others, availability of water, minerals and nutrients, temperature, light intensities, presence of competitors or pathogens, and occurrence of soil or air pollution. In agriculture, suboptimal growing conditions can often be remedied. For example, dry soils are irrigated, poor soils are fertilized, while pesticides and herbicides are applied to control pathogen infestations and competitors, respectively. Yet, the growing concern for sustainable and environmental friendly agriculture demands for changes in farming practices. Massive irrigation of farmland, commonly used for the cultivation of cotton and other crops, are being increasingly opposed because they lead to salinization of soils and a reduction in water levels in downstream areas. Similarly, the intensified use of agrochemicals is heavily criticized, because of suspected negative effects on the well being of humans and animals. At the same time, the growing world population is forcing agriculture into the use of marginal land, thus expanding the range of environments in which crops are cultivated. As a result, the production of stress-tolerant varieties has become a worldwide priority for most important crops.
Although conventional plant-breeding programs have improved yields for crops grown in stressful environments, there is a growing belief that further gains will mostly be achieved through targeted manipulation of genes involved in stress tolerance. Many stress-inducible genes have been identified over the past years, some of which were shown to confer a certain increase in stress tolerance, when overexpressed in transgenic plants. However, from these studies the notion emerged that tolerance to environmental stress is highly complex, requiring the coordinated activation of multiple genes. This has led to the adoption of transgenic strategies that make use of signal transduction components controlling the expression or activity of stress defense proteins, rather than of stress defense proteins themselves.
Successful examples of this kind are the overexpression of AP2 domain transcription factors CBF1 and DREB1A, and of the heat-shock factors HSF1 and 3 in Arabidopsis. CBF1 was shown to enhance freezing tolerance (Jaglo-Ottosen et al., Science 280:104-106, 1998; Thomashow, U.S. Pat. No. 5,929,305), while DREB1A induced tolerance to cold and drought stress (Kasuga et al., Nature Biotechnol 19:287-291, 1999) in Arabidopsis. HSF1 and 3 both conferred thermotolerance in transgenic plants (Lee and Schöffl, Plant J 8:603-612, 1995; Prandl et al., Mol Gen Genet 258:269-278, 1998).